The Prevention and Elimination of Grain Inheritance in Forging
Introduction
Grains inheritance has been found to be one of the major causes of defects in forged parts. It occurs during the deformation process of the work material when internal stress is introduced, leading to poor grain structure and increased internal stress levels in the finished part. The importance of preventing and eliminating grain inheritance in forging processes cannot be overstated: it is expensive, time-consuming, and has a huge impact on the quality of the part.
This paper presents the current research on preventing and eliminating grain inheritance in forging processes, with a focus on the various methods used to reduce its occurrence and the effects of different materials on grain inheritance.
Grains Administration
Grain administration can be described as the systematic control of grain size and structure during the forging process to prevent the inheritance of defects. The process itself involves the optimization of heating, hammering and forcing (strain) applied to the work piece.
Generally, the heating condition is optimized to ensure the homogeneous transformation of austenite and pearlite alloys, providing uniform grain size and morphology, and the hammering speed affects the maximum grain size and grain morphology. In the forcing stage, strain planning, including strain types and combinations, is used to minimize the inheritance of internal stress levels and prevent cracking.
Grain redefining is another key process related to grain administration. It consists of cyclostationary cold working, annealing and tempering, so-called tempering cycles. This can be used to regenerate the grains into the desired size and morphology, allowing the part to be formed with minimal to no internal stress levels.
Material Selection
The previous section discussed the effects of strain on grain inheritance. It is also important to consider the effects of different materials on grain inheritance. Alloying element addition to the base material is one way of controlling grain size and structure. The addition of nickel and manganese, for example, is used to increase the grain size. On the other hand, titanium, molybdenum, and aluminum alloys have been used to reduce the grain size.
Materials can also be divided into two categories: grain refiners (GRs) and grain inhibitors (GIs). GRs are components that cause a small increase in grain size, while GIs cause a large decrease in grain size. The combination of both categories of materials can be used to optimize grain structure.
Inhibiting Factors
An important factor in preventing and eliminating grain inheritance is understanding the factors that contribute to it. These factors include: surface roughness, oxidation, strain hardening, plastic deformation, and cold work.
Surface roughness affects how the material deforms. The more internal defects a material has, the more difficult it is to deform. Oxidation can also affect grain size, as oxide layers can impede deformation. Strain hardening increases the internal stress in the material, which can result in grain inheritance. Plastic deformation occurs when the material is hammering and forcing, resulting in localized grain size and morphology change. Cold work is the final factor, affecting the grain size and morphology due to the accumulation of strain.
Conclusion
Grain inheritance is a common issue in forged parts and can lead to decreased part quality and expensive rework processes. The prevention and elimination of grain inheritance in forging processes requires an understanding of the grain administration process and the effects of different materials on grain size and structure. The combination of heating, hammering and forcing strain must be optimized in order to achieve uniform grain size and morphology. Furthermore, grain redefining and cold working can be used to regenerate grains into the desired size and morphology. Finally, it is important to understand the factors that contribute to grain inheritance in order to adequately prevent and eliminate it.
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